This invention relates to a videodisc reproducing apparatus for reproducing videodiscs on which band-compressed signals of high-definition television called MUSE are recorded.
Now is proposed a method of band-compressed transmission called MUSE for transmitting video signals of HDTV capable of obtaining higher definition pictures as compared with the existing standard television system by using a channel of 27 MHz bandwidth of satellite broadcasting in Japan.
This transmission method is initially intended to realize satellite broadcasting in the same transmission channel as the existing broadcasting, by compressing the HDTV signals possessing a bandwidth of 20 MHz into a base band of 8 MHz by multiplex sub-Nyquist sampling, and then frequency-modulating to suppress the transmission band to a width of a 24 to 27 MHz corresponding to one channel of broadcasting satellite.
In this method, as stated above, the band is compressed to 1/4 by 4:1 sub-sampling. This sampling pattern is shown in FIG. 1. In the diagram, the solid line 1 denotes scanning lines of 2 n fields (n is a positive integer), and the broken line 2 represents scanning lines of 2 n+1 fields. Numeral 3 indicates a sampling point of the 4 n-th field; numeral 4 is a sampling point of the 4 n+1-th field; numeral 5 is a sampling point of the 4 n+2-th field; numeral 6 is a sampling point of the 4 n+3-th field, and numeral 7 indicates a point not transmitted. The image sampled in this manner is compressed to 1/4 of the original band, and is transmitted.
On the other hand, at the receiving side, there is a frame memory, and all sampling points of the portion of four fields being transmitted are utilized to reproduce the image, and furthermore at the point 7 in FIG. 1 that is not transmitted, the image is reproduced by two-dimensional interpolation. In this way, a still picture can be perfectly reproduced into the original image, but in the moving picture, since the correlation between fields is low, when the points of all four fields are used, the image may be blurred or the sampling pattern may be visible, and the picture is disturbed. Therefore, for reproduction of a moving picture, it is necessary to reproduce only from, the sampling points of one field being transmitted at the present. Accordingly, as a matter of course, the transmission signal band of the moving picture portion becomes narrow, and blurring becomes obvious, and as its countermeasure, when moving the entire picture parallel such as panning, the motion vector is transmitted, and the sampling point position in interpolation between frames is corrected at the receiving side so as to interpolate accurately. In this method, in the case of parallel moving of the entire picture such as panning, in spite of the moving picture, a high resolution similar to that of a still picture can be obtained.
As clear from the description herein, this technique is a band compression method of high definition television signals suited to both still pictures and moving pictures, but various control signals such as correction position information of sampling points relating to still pictures and moving pictures are transmitted once in one field as being superposed as digital signals within the vertical blanking period, and, at the receiving side, the sampling point positions are corrected by controlling the writing and reading addresses of the frame memory according to these control signals.
The signal form in the state of the baseband according to this method is explained by reference to FIG. 2, in which numerals 11 and 12 are horizontal synchronizing signals, 11 being rising and 12 being falling, inverting in every one horizontal scanning period (expressed 1H hereinafter). Numeral 13 represents a brightness signal period which was originally a brightness signal of 20 MHz bandwidth and compressed in bandwidth to 8 MHz by sub-Nyquist sampling, and 14 is a color difference signal period which had originally two color difference signals called R-Y, B-Y as line sequential signals and was subjected to sub-Nyquist sampling to a frequency band of 8 MHz with the time axis compressed to 1/4.
The vertical synchronizing signal portion is explained with reference to FIGS. 3A-3B. FIGS. 3A-3B respectively shows the portion of 2H of the first line and second line of the scanning lines composing one screen of 1125 lines. Numeral 15 denotes a pulse train repeated 17.5 times in 4 transmission clock widths, and 16 is a flat period of 16 transmission clock widths; after this the level is inverted and there is a flat period 17 of 18 transmission clockwidths. The signals indicated by 15 to 17 are presented as being inverted on the first line and second line. Numeral 18 is a horizontal synchronizing signal, and it was originally inverted between rising and falling in every 1H, but since the signal is reset on the second line, the same falling signals continue as shown in the diagram. The changeover point 19 of 16 and 17 on the second line is the frame pulse point, which corresponds to the conventional vertical synchronizing signal. In this example, however, unlike the conventional vertical synchronizing signal, it exists only once in one frame.
After band compression, synchronizing signals as explained above are added to the MUSE signals, which are actually emphasized or dispersed for energy dispersion before frequency modulation, and are transmitted through a broadcasting satellite.
On the other hand, by this method, since the high definition television signals are compressed in bandwidth from over 20 MHz to about 8 MHz, it is useful not only for satellite broadcasting, but also for recording and reproducing, and it is expected to be applied to VCR's, and videodiscs, etc.
As for the videodisc, in particular, since the high definition, wide and dynamic pictures can be readily enjoyed at home, an early commercialization is expected. The greatest difference of this MUSE system videodisc from the existing television system is that the synchronizing signals cannot be easily separated in amplitude because of positive pole synchronization as shown in FIGS. 2 and 3A-3B, and therefore it is very difficult, if not impossible, to use the synchronizing signals by separating them in the control of rotation of the disc, etc. It is accordingly proposed to record pilot carrier signals which have a single continuous frequency synchronized with the video signals on a disc by frequency-multiplexing on frequency-modulated MUSE signals, and detect these pilot carrier signals when reproducing to use them in the control of disc.
As explained hereabove, since the MUSE signal is composed of sub-Nyquist sampling, if there is jitter in the reproduced signal, a deviation occurs in the sample phase, and it may be impossible to decode into the original signal, and hence it is necessary to shift the time axis or correct the jitter. In the prior art, the jitter was mechanically corrected by detecting the pilot carrier signals and directly driving the reproduction pickup of the disc, or shifting the mirror fixed to the galvanometer.
Thus, in the MUSE system videodisc, correction of jitter is important, but in order to spread widely in the consumer product market, a smaller size and a lower price of the machines are essential, and there is a limit in the mechanical jitter correction method. Accordingly, an electrical method may be considered, and from the viewpoint of quality of reproduced signals, a digital time base collector (TBC) using a digital memory is normally used. Since the television signal of the MUSE system possesses a bandwidth of 8 MHz or more, in order to convert some into a digital signal without sacrificing the reproduction bandwidth, it is generally necessary to convert the signal from an analog signal to a digital signal (A/D conversion) at a sampling clock of 24 MHz or more. The frequency of this level is, however, near the limit of ordinary TTL logic IC's, and a particularly high speed TTL IC is needed. But high speed TTL IC's have a high in power consumption, or when parallel processing is done in order to use low speed IC's, it gives rise to an increase in the number of circuit elements, which was a great barrier for realizing low cost LSI's for consumer use.